Organic electroluminescent devices (OEDs), and especially organic light emitting diodes (LEDs) and the like, generally utilize a layer of reactive metal in the cathode to ensure efficient injection of electrons and low operating voltages. However, reactive metals are particularly susceptible to damage due to oxidation in the presence of oxygen and moisture. Oxidation of the metal severely limits the lifetime of a device. A hermetic seal is normally required to achieve long term stability and longevity. For example, the time to half the luminance intensity under constant current condition for an unencapsulated device versus an encapsulated device is 200 hours as opposed to 1000 hours. Also, the rate of dark spot growth is significantly reduced for encapsulated devices.
Several approaches to protecting or encapsulating OEDs have been reported. In a prior art (JP 91359134), a glass cover was directly adhered to the OED active area and device substrate with a photo curing resin layer. Another prior art (U.S. Pat. No. 5,189,405), used a series of sheets inside a cover laminate consisting of a metallic foil between two organic resin films adhered to the OED glass substrate using an epoxy resin. Typically, a low temperature, thermally cured organic epoxy material is used to create the perimeter seal or barrier and adhere the cover to the device substrate, and is usually accomplished by curing the organic epoxy at 85 degrees Celsius over a two hour period. Furthermore, the permeation rate of oxygen and water through organic barriers is several orders of magnitude greater than through inorganic barriers. Thus, the organic epoxy provides a path for the ingress of oxygen and water. A further problem is a result of the fact that the organic layers, particularly the hole transport materials are sensitive to temperatures as related to their property of glass transition. The glass transition is best described as a molten solid, and for many organic materials occurs at relatively low temperatures, such as below 100 degrees Celsius. The potential for the organic material to crystallize is greater once the glass transition has been reached. Crystallization of an organic film in the fabrication of an OED leads to low charge mobility, charge traps, pinholes and shorts. In many instances, even approaching the critical temperatures of the organic layers, especially if the elevated temperatures are maintained for relatively long periods of time, can degrade the organic material and reduce the reliability and/or the longevity of the device.
Several integrated approaches have also been developed. The integrated approaches typically include a protection layer deposited on the discrete device as part of the fabrication process. The integrated encapsulation schemes require specialized processing equipment and techniques.
Accordingly, it is highly desirable to devise a relatively inexpensive and convenient encapsulation package for hermetically sealing organic electroluminescent devices.
It is a purpose of the present invention to provide a new and improved encapsulation package for organic electroluminescent devices.
It is another purpose of the present invention to provide a new and improved encapsulation package for organic electroluminescent devices which has a thin profile and is light in weight.
It is a further purpose of the present invention to provide an encapsulation package for organic electroluminescent devices which acts as an effective barrier to oxygen and moisture.